1. Field of the Invention
This invention relates in general to spread spectrum receivers, more specifically to terrestrial GPS navigation systems such as those used in terrestrial navigation for cars, trucks and other land vehicles, and in particular to terrestrial GPS receivers used in environments including substantial multipath interference, such as urban environments.
2. Description of the Prior Art
GPS systems, as well as many other radio frequency (RF) communication systems utilizing frequencies high enough to be considered line of sight systems in which there must be a substantially direct line of sight between the transmitter(s) and receivers(s) for optimum operation, often suffer from multipath effects in which the receiver(s) must process signals received over a multiplicity of different paths. A common example is a simple broadcast TV system in which a TV receiver with an antenna receives multiple copies of the signal being transmitted.
The multiplicity of signals being received results from additional, typically unwanted, signals paths including one or more reflections. When the signal path from the transmitter to receiver includes a reflection, this signal path must by definition be longer than the direct path. Multipath signals present a problem in systems, such as GPS systems, in which the time of arrival of the signal is to be measured or used because the time of arrival of the multipath signals depends on the length of the path(s) taken.
The straightforward processing of all signals, including multipath or reflected signals, often degrades the processing performed by the receiver. The direct path is the shortest and therefore requires the least travel time from transmitter to receiver while the various unwanted multipath signals have various greater lengths, and therefore various longer travel times, than the direct path signals.
GPS transmitters are positioned on satellites with complex orbital paths so that the position of the multiple transmitters are constantly changing. This makes a highly directional antenna system almost completely unusable. Similarly, digital receivers, including those used in a GPS receiver, often do not rely solely on the amplitudes of the signals received, but rather rely on other signal characteristics, such as time of arrival.
Multipath processing techniques currently used for complex receivers, such as GPS receivers, are often quite complex and subject to inaccuracies. An example of one such conventional technique is described in U.S. Pat. No. 5,414,729 issued on May 9, 1995 to Patrick Fenton and assigned as issued to NovAtel Communications Ltd., Canada. In this technique, an autocorrelation function of a partially processed received signal, including multipath components, is compared to an estimated autocorrelation function of an estimated direct path signal to attempt to discern direct path signals from muitipath signals for further processing. This technique of comparing processed and estimated correlation power, is complex and may be subject to error in that the partially processed signals relied on are themselves subject to degradation from many effects in addition to multipath effects including receiver limitations, which may reduce the accuracy or effectiveness of the multipath processing techniques.
For example, in tracking a GPS C/A signal to determine position information from GPS satellite transmitters, it is typically important to derive an accurate estimate of the time of arrival, known as code phase, of the PRN modulation of the direct path component of the C/A signals received from each of the various GPS satellites. It is also important to derive an accurate estimate of the phase of the underlying carrier signals transmitted from the satellites on which the modulation is applied, known as the carrier phase. However, as apparently shown for example in FIGS. 6, 7 and 8 of the above referenced Fenton patent, the delayed multipath components degrade the tracking of the code and carrier phase estimates by distorting the correlation functions used is such tracking.
What is needed is an improved spread spectrum receiver, such as one for use with GPS navigation systems, which avoids the limitations of conventional designs and provides improved results in a wide range of reception conditions, including multipath interference.